1. The Field of the Invention
The present invention relates to explosives, and more particularly, to dry explosives for use in developing surface seismic shock energy.
2. The Prior Art
Surface seismic shock explosives have been widely accepted as a preferred means of inducing shock waves into the earth. The shock waves, according to known techniques, are reflected by subsurface geological strata and detected again at or near the surface of the earth using detectors, such as geophones. Analysis of the reflected shock waves permits skilled analysts to gain valuable geological information which, among other uses, assists in the discovery of subsurface gas and oil.
The explosive composition which is used to induce the shock wave into the earth must develop the necessary shock energy, while at the same time permit safe and facile handling. Several prior art products have been developed to address these characteristics. Examples of such prior art products are "Thermex, " sold by Thermex Energy Corporation, and "Surf-a-seis," sold by Hercules, Incorporated.
Prior art compositions of dry explosive are typically packaged in a flexible plastic bag which is suspended upon a ground stake and tied with high grain (25 to 30 grains of PETN per foot) detonating cord. The detonating cord initiates the seismic explosive, which in turn generates the shock wave.
Typically, seismic explosives are used in field environments which give rise to serious concern about fire. In the typical circumstance, plastic containers of seismic explosives are placed on wooden stakes and spaced in an array at a site to be tested. It is not uncommon for the site to be covered with dry grass and foliage which creates a serious fire hazard when seismic explosives are used. One contributor to the fire hazard is the high grain detonating cord which is required to initiate many of the prior art seismic compositions.
To minimize the fire hazard, some prior art products are sold with a pouch of fire retardant to be placed on the positioning stake below the explosive prior to initiation. When the explosive shoots, the retardant is designed to quench any fire which is ignited. The retardant, however, increases the cost and the complexity of seismic exploration.
The prior art compositions each use an oxidizer and a fuel mixed together with a sensitizer, such as finely divided aluminum. However, finely divided (paint grade) aluminum is expensive and cannot be used alone reliably so as to be both safe in handling and reliable in initiating. Accordingly, prior art compositions typically include a Nitroparaffin sensitizer, such as 1-Nitropropane, which are liquids with high vapor pressures. When carefully mixed with the dry fuel and oxidizers, Nitroparaffins can effectively increase the sensitivity.
Liquid Nitroparaffins, however, used in the prior art compositions disadvantageously tend to vaporize under higher temperature conditions, thereby causing the plastic bag containing the composition to swell. Thus, special care and expense must be invested in each packaging bag to assure that margins and openings are sealed against the increased vapor pressure inside the bags. Unless the bag is specially sealed, it will rupture and the Nitroparaffin will evaporate.
It would, therefore, be a significant imporvement in the art to provide a dry surface seismic composition which reliably initiates with lower grain detonating cord and which is also safe and easy to handle without the attendant problems presented by the prior art compositions. Such an explosive composition is disclosed and claimed herein.